Quantitative investigation of internal polarization in a proton exchange membrane water electrolyzer stack using distribution of relaxation times

Proton exchange membrane water electrolyzer (PEMWE) is a promising technology for hydrogen production due to its ability to operate at high currents, compact design, and high produced hydrogen purity. However, the high cost and limited durability challenges must be addressed to advance the commercialization of PEMWEs. Accessing the internal polarization processes is crucial to understanding the performance of PEMWEs and guiding their design and operation. In practice, the output voltage amplitude on a specific current value is often considered a performance indicator. However, PEMWEs are complex systems with multiple polarization processes that are inaccessible using global indicators such as voltage. We propose a distribution of relaxation times (DRT) based approach to overcome this challenge. DRT is a model-free method that deconvolutes the electrochemical impedance spectroscopy data into a series of relaxation times, corresponding to different internal polarization processes. The results show that the internal polarization processes of the PEMWE can be decomposed into four peaks, corresponding to proton transport in the ionomer of catalyst layer, charge transfer during oxygen evolution reaction and hydrogen evolution reaction, and mass transport. The contribution of these processes and high-frequency resistance (HFR) to the overall overpotential losses are further quantified, which indicates that HFR (79.4%) and charge transfer (16.4%) are the two dominant factors. Finally, the influence of operating temperature and cathode pressure on the performance of the PEMWE is quantified using the proposed approach. This approach can be generalized to identify the degradation root cause of PEMWEs which can guide material enhancement and operation optimization to improve the efficiency and durability of PEMWEs.